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I n d . E n g . Chem. R e s . 1989, 28,438-444
Studies in a Moving Bed Pressure Gasifier: Prediction of Reaction Zones and Temperature Profile? T. Krishnudu,* B. Madhusudhan, S. Narayan Reddy, V. S. R. Sastry, K. Seshagiri Rao, and R. Vaidyeswaran Regional Research Laboratory, Hyderabad 500 007, India
Sequential layer samples were collected from a moving bed pressure gasification pilot plant operating at 2.0 MPa after i t was quenched by cutting off the supply of oxygen and steam, and the samples were then analyzed. The studies were made with two low-rank coals. Based on the proximate and ultimate analyses and Gray-King assay, three zones-fast devolatilization, slow devolatilization with gasification, and gasification-could be indicated in the gasifier. From a correlation obtained earlier between volatile matter content in the char and the temperature of low-temperature carbonization in a Lurgi Spuelgas type of reactor, the temperature profile in the gasifier was predicted using the volatile matter content in the sequential layer samples. About two-thirds of the total fuel bed height of 3500 mm was for fast and slow devolatilizations and the rest for gasification. Gasification of coal is a complex process involving a number of reactions between coal and the reactants oxygen and steam and between the products carbon dioxide and hydrogen and coal. In addition, secondary reactions in the gas phase among the gaseous products also take place. A knowledge of the various reactions taking place during gasification will enable optimization in the design of the gasifier to achieve maximum conversion to useful products. Rudolph (1972) reported that during gasification coal undergoes processes like drying, devolatilization, gasification, and combustion as it descends in a moving bed gasifier, finally leaving ash. Denn and Shinnar (1987) have referred to the existence of four zones in a moving bed gasifier with countercurrent movement of coal and gaseous reactants and products. These are the drying, devolatilization, gasification, and combustion zones. These four zones are definable but not distinctly separable. No precise information is available on the extent of these zones or the temperature profile in the gasifier. For a proper design of the gasifier, information on the extent of the zones is necessary to fix the height of each of these zones and to determine the total height of the gasifier. Attempts have been made to measure the temperatures inside the gasifier. Hebden et al. (1954) measured the axial temperatures in a 2-ft-diameter gasifier using different types of fuel and different pressures and steam-oxygen ratios. Temperature measurements and gas samples were taken at various levels in the bed and under varying operating conditions. However, the different zones were not demarcated in these studies. According to Johnson and Thomas (1987), use of thermocouples to measure the temperature inside a moving bed gasifier suffers from the disadvantage that the thermocouples tend to get damaged by the flow of coal and they may perturb the local flow of char and gas. In their study they collected "bed dig out" samples by sudden quenching of an operating gasifier and used reflectance measurement and Raman microprobe spectroscopy for estimation of heat treatment temperature and thermal heterogeneity in char samples. The results gave a thermal history of the sample, but the temperature profile in the gasifier was not indicated. In the present study, the method of characterization of char samples by its volatile matter content and correlation of it with the carbonization temperature has been used to estimate the temperature profile and to indicate the different zones in the gasifier. *Author t o whom all correspondence should be addressed. RRL(H) Communication No. 2179.
Sequential layer samples were drawn from a Lurgi dry ash moving bed pressure gasifier after it was quenched by cutting off the supply of oxygen and steam, and the samples were analyzed. The present paper gives the results of this study.
Experimental Procedure The gasification pilot plant has a coal throughput of 1 tonne/h and can operate at pressures up to 2.4 MPa. The flow sheet of the pilot plant is given in Figure 1. The main gasifier is a water-jacketed vessel with an internal diameter of 1.13 m and an external diameter of 1.3 m. Facilities exist for feeding coal and withdrawing ash under pressure by lock hopper systems. The gasifying medium, steamoxygen mixture, is introduced from the bottom through the ash discharge grate. The ash collected at the bottom of the gasifier is withdrawn by means of the grate, which can be rotated at different speeds. This along with the rate of supply of oxygen and steam helps to control the throughput of the gasifier. The gasifier has not been provided with a coal distributor. Instead, a chute has been installed to direct the flow of coal and provide the gas collecting space. For noncaking coals, the coal distributor is not required. The gas cooling and condensation sections consist of a scrubber-cooler, a waste heat boiler, and an aftercooler. An air separation plant for the production of 250 m3Nof oxygen, a high-pressure steam boiler to generate 2.5 tonne/h of steam at 400 "C and 3.3 MPa, a coal feed preparation plant, an effluent treatment plant, and a boiler feed water preparation plant are the other facilities available. When the gasifier was processing a low-rank, noncaking coal from Godavari Khani mine (Ramagundam colliery), Godavari Valley coalfield, at a pressure of 2 MPa, it was quenched by cutting off the supply of oxygen and steam, in that order. The gasifier was depressurized gradually to atmospheric pressure, by which time the flow of gas ceased. The operating conditions and other data at the time of quenching are given in Table I. The gasifier was allowed to cool, and the temperatures of the gas leaving the gasifier and the waste heat boiler were observed at regular intervals of time until ambient conditions were reached. Sequential layers of the bed, each corresponding to 250 mm thickness along the height of the gasifier and equivalent to a volume of 0.25 m3, were withdrawn by operating the ash grate. The volume around the grate amounted to 0.25 m3 and formed the first sample. According t~ an Indian Standard method (1964) for sampling 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 439 STACK
r
Figure 1. Flow sheet of gasification plant. Table I. Process Conditions at the Time of Quenching coal Godavari KhaniManuguruRamagundam Kothagudem coal feed rate, kg/h 850 890 steam flow, kg/ h 948 790 oxygen flow, m 3 ~ / h 155 157 blast temp, O C 310 330 moisture in coal, w t % 4.2 2.5 operating pressure, MPa 2.0 2.0 crude gas flow, m 3 ~ / h 1020 1060 gas outlet temp, OC 350 360 gas composition, vol % 33.0 30.8 COZ 17.8 17.6 co H2 38.0 40.8 9.8 9.0 CHI 0 2 0.6 0.6 N2 (by diff.) 0.8 1.2
of coal for the proximate and ultimate analyses and Gray-King assay on samples of size 6-25 mm, a minimum quantity of about 75 kg is required. For carrying out this analysis as well as for determination of physical properties, it was considered necessary to collect samples of about 150-180 kg (or about 0.25 m3) each. Each sequential layer sample thus corresponded to a volume of 0.25 m3 and a height of 250 mm in the gasifier. In all, 14 sequential layer samples designated as 1-14 were collected, the top-most layer being 14. Their locations with respect to gasifier height are shown in Figure 2. These samples were subsampled and analyzed for proximate and ultimate analyses according to Indian Standard methods (1969, 1974) and Gray-King assay according to an Indian Standard method (1960). Physical properties like bulk density, true density, screen analysis, and average particle size were determined by the usual methods. A representative sample was also collected from the coal fed to the gasifier and analysis sample prepared. GrayKing assay was done with this sample between 400 and 900 "C at intervals of 100 "C, and proximate analysis was done on the chars thus obtained.
OUTL El
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Figure 2. Position of each sequential layer sample in the gasifier.
The quenching studies were repeated with another low rank noncaking coal from Manuguru colliery, Kothagudem also in Godavari Valley coalfield. The conditions and gas composition a t the time of quenching with this coal are also given in Table I. With this coal, the content of carbon dioxide after the supply of steam and oxygen was cut off was 27.6%, and it decreased to 25.2% at the end of 2 h. During depressurization, oxygen content remained in the range 0.4-0.6%. The temperature of gas leaving the gasifier and the waste heat boiler during the cooling of the
440 Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 Table 11. Analyses of Sequential Layer Samples from Quenching Experiments: Godavari Khani-Ramagundam Coal sample 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 prox. anal., wt % (air-dried basis) moisture 1.0 1.8 1.8 1.2 2.2 1.9 0.9 1.1 0.8 1.9 2.4 3.2 4.6 ash 85.4 69.5 64.2 60.0 58.9 58.0 56.0 52.0 48.6 46.6 35.5 34.7 32.1 volatile matter 2.4 3.1 2.7 3.4 3.4 4.7 4.8 5.2 5.9 6.1 17.5 21.6 22.9 fixed C 11.2 25.6 31.3 35.4 35.5 35.4 38.3 41.7 44.7 45.4 44.6 40.5 40.4 ult. anal., wt % (air-dried basis) moisture 0.95 1.84 1.84 1.18 2.17 1.93 0.89 1.12 1.26 1.87 2.42 3.20 4.60 ash 85.40 69.53 64.23 60.01 58.91 57.96 56.04 52.03 48.61 46.55 35.50 34.70 32.00 12.95 27.38 30.84 34.16 35.64 36.56 39.03 43.63 45.10 46.08 49.70 48.94 49.45 C 0.44 0.76 0.7% 0.71 1.04 0.98 1.00 1.29 1.30 1.30 2.33 H 2.64 2.21 Gray-King assay at 600 OC (yields per 100 g of dry coal) 99.23 98.95 98.71 98.28 97.90 97.65 97.86 97.18 97.14 96.68 86.72 82.68 81.75 coke, g nil nil nil 0.83 nil 1.31 0.75 0.98 0.92 0.94 4.80 5.33 5.01 tar, g nil nil nil nil 0.24 0.21 0.37 0.50 0.54 0.52 3.11 5.28 5.52 liquor, g 0.77 1.05 1.10 1.17 0.83 gas, L a t NTP 1.10 0.96 1.84 1.83 2.19 6.48 6.69 7.73
79.98 5.50 6.25 9.25
Table 111. Physical Properties of Sequential Layer Samples from Quenching Experiments: Godavari Khani-Ramagundam Coal sample 1 2 3 4 5 6 7 8 9 766 847 830 815 823 801 812 777 780 bulk density, kg/m3 1879 1826 1781 1732 1680 1572 1572 1577 1512 true density, kg/m3 screen anal., wt 70 retained on 0.97 0.38 0.49 0.67 0.78 0.69 3.07 0.51 0.92 25 mm 7.19 5.40 7.12 7.28 10.63 5.68 7.08 6.25 8.37 13 mm 42.00 44.74 49.58 44.78 53.96 46.82 54.20 52.84 58.75 6 mm 60.34 63.87 64.88 61.06 67.60 57.72 71.63 70.43 71.22 3.35 mm 73.39 76.16 75.48 74.01 77.24 69.32 83.16 80.53 80.19 2.00 mm 83.18 84.44 83.70 74.14 84.80 79.68 90.25 87.55 86.77 1.00 mm 83.18 84.55 83.78 84.55 84.88 79.86 90.41 87.62 86.90 0.85 mm 85.63 85.78 85.19 85.76 86.06 81.38 91.11 88.88 88.18 0.60 mm 89.40 89.41 88.98 89.69 89.76 86.48 93.55 91.73 91.77 0.40 mm 92.66 92.51 91.96 93.06 92.66 90.42 95.50 94.55 94.59 0.20 mm 94.55 94.06 93.37 94.40 93.82 92.59 96.26 95.53 95.73 0.15 mm 4.27 5.45 5.94 6.63 7.41 3.74 4.47 5.60 6.18
